$ { {-4} \times \left[\begin{array}{rr}{-1} & {3} \\ {0} & {3} \\ {0} & {4}\end{array}\right] = {?} } $
Explanation: To multiply a matrix by a number, multiply each item of the matrix by the number. ${\left[\begin{array}{rr}{-4}\times{-1} & {-4}\times{3} \\ {-4}\times{0} & {-4}\times{3} \\ {-4}\times{0} & {-4}\times{4}\end{array}\right]}$ Now, simplify: ${\left[\begin{array}{rr}{4} & {-12} \\ {0} & {-12} \\ {0} & {-16}\end{array}\right]}$